Method and apparatus for insitu vapor generation

ABSTRACT

A method of forming an oxide on a substrate. According to the method of the present invention a substrate is placed in a chamber. An oxygen containing gas and a hydrogen containing gas are then fed into the chamber. The oxygen containing gas and the hydrogen containing gas are then caused to react with one another to form water vapor in the chamber. The water vapor then oxidizes the substrate.

[0001] The present application is a Continuation-in-Part of prior U.S.patent application Ser. No. 08/893,774, filed Jul. 11, 1997 and assignedto the present Assignee.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to the field of steam oxidation andmore specifically to a method and apparatus for insitu moisturegeneration in a rapid thermal steam oxidation process.

[0004] 2. Discussion of Related Art

[0005] In the fabrication of modem integrated circuits, such asmicroprocessors and memories, oxidation processes are used to passivateor oxidize silicon films. A popular method to oxidize silicon surfacesand films such as polysilicon gate electrodes and substrates is to use asteam (H₂O) oxidation process. In such cases water vapor (H₂O) isbrought into an oxidation chamber to react with the silicon surfaces toform silicon dioxide.

[0006] Present steam oxidation processes generally take place inmulti-wafer resistively heated “hot wall” furnaces. Present steamoxidation processes typically use a pyrogenic torch or bubbler locatedoutside of the reaction chamber in which the steam oxidation process isto take place. In the case of a pyrogenic torch, a hydrogen containinggas and an oxygen containing gas are ignited by a flame in a reactionarea at atmospheric pressure and located away from and generally in adifferent chamber than the chamber in which wafers are placed. The flameignition occurs at atmospheric pressure. A problem associated withpyrogenic torch methods, is that for safety reasons only certainconcentration ratios of hydrogen containing gas and oxygen containinggas can be utilized. Limiting the available gas ratio unduly restrictsones ability to generate ambients with desired concentrations of H₂O/H₂or H₂O/O₂. For example, in order to keep a stable flame burning, torchmethods typically require H₂:O₂ ratios of more that 0.5:1 and less than1.8:1, respectively. Bubblers are also undesirable for moisturegeneration in that they can be a significant source of contamination andbecause they cannot accurately and reliably control the amount ofmoisture generated.

[0007] Another problem associated with the use of pyrogenic torches andbubblers is that these methods are not easily implemented into modernrapid thermal heating apparatuses which utilize light sources for rapidtemperature ramps and reaction times measured in terms of seconds asopposed to minutes and hours. Rapid thermal heaters are preferred overresistively heated furnaces because of their excellent temperatureuniformity and control provides more for uniform processing and becausetheir short reaction times reduce the thermal budget of fabricateddevices.

[0008] Thus, what is desired is a method and apparatus for generatingmoisture in a rapid thermal heating apparatus which does not suffer fromcontamination and safety issues and which can use a full spectrum of gasmixtures as well as concentration ratios.

SUMMARY OF THE INVENTION

[0009] A method of forming an oxide on a substrate is described.According to the present invention a substrate is placed in a reactionchamber. An oxygen containing gas and a hydrogen containing gas are thenfed into the chamber. The oxygen containing gas and the hydrogencontaining gas are then caused to react in the chamber to form watervapor. The formed water vapor is used to oxidize the substrate. In anembodiment of the present invention a combined partial pressure of theoxygen containing gas and the hydrogen containing gas in the reactionchamber is developed between 1-50 Torr to enhance the oxidation rate ofsilicon by the water vapor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is an illustration of a rapid thermal heating apparatuswhich can implement the insitu moisture generation oxidation process ofthe present invention.

[0011]FIG. 2 is an illustration of the light source placement in therapid thermal heating apparatus of FIG. 1.

[0012]FIG. 3 is a flow chart which illustrates a rapid thermal oxidationprocess which utilizes the insitu moisture generation process of thepresent invention;

[0013]FIG. 4a is a cross sectional view of a semiconductor wafer orsubstrate prior to steam oxidation.

[0014]FIG. 4b is an illustration of a cross sectional view showing theformation of an oxide on the substrate of FIG. 4a by a rapid thermaloxidation process which utilizes insitu moisture generation of thepresent invention.

[0015]FIG. 5 is a graph which illustrates the detonation pressurecreated for various O₂/H₂ concentration ratios having a partial pressureof 150 torr.

[0016]FIG. 6 illustrates plots which depict oxide thickness versusreactant gas partial pressure for different H₂/O₂ concentrations.

[0017]FIG. 7 is a plot which illustrates oxide thickness versus H₂/O₂reactant gas concentration ratios.

[0018]FIG. 8 illustrate plots which depict oxide thickness versusoxidation time for various concentration ratios and reactant gas partialpressures.

[0019]FIG. 9 is a plot which illustrates oxide thickness versus totalflow of process gas.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0020] The present invention describes a novel method and apparatus forinsitu moisture generation in a rapid thermal steam oxidation process.In the following description numerous specific details such as apparatusconfigurations as well as process specifics such as time and temperatureare set forth in order to provide a thorough understanding of thepresent invention. One skilled in the art will appreciate the ability touse alternative configurations and process details to the disclosedspecifics without departing from the scope of the present invention. Inother instances well known semiconductor processing equipment andtechniques have not been described in detail in order to notunnecessarily obscure the present invention.

[0021] The present invention describes a steam oxidation process.According to the present invention, steam (H₂O) is formed in the samechamber as which the substrate to be oxidized is located (i.e., steam isformed insitu with the substrate). According to the present invention areactant gas mixture comprising a hydrogen containing gas, such as butnot limited to H₂ and NH₃ and an oxygen containing gas, such as but notlimited to O₂ and N₂O, is fed into a reaction chamber in which asubstrate is located. The oxygen containing gas and the hydrogencontaining gas are caused to react to form moisture or steam (H₂O) inthe reaction chamber. The reaction of the hydrogen containing gas andthe oxygen containing gas is ignited or catalyzed by heating the waferto a temperature sufficient to cause the moisture reaction. Because theheated wafer is used as the ignition source for the reaction, themoisture generation reaction occurs in close proximity to the wafersurface. Reactant gas concentrations and partial pressures arecontrolled so as to prevent spontaneous combustion within the chamber.By keeping the chamber partial pressure of the reactant gas mixture atless than or equal to 150 torr during the reaction, any reactant gasconcentration may be utilized to form moisture without causingspontaneous combustion. The insitu moisture generation process of thepresent invention preferably occurs in a reduced pressure single waferchamber of a rapid thermal processor. A rapid thermal steam oxidationprocess utilizing insitu moisture generation is ideally suited foroxidizing a silicon film or substrate in the formation of modern ultrahigh density integrated circuits.

[0022] The insitu moisture generation process of the present inventionis preferably carried out in a rapid thermal heating apparatus, such asbut not limited to, the Applied Materials, Inc. RTP Centura with aHoneycomb Source. Another suitable rapid thermal heating apparatus andits method of operation is set forth in U.S. Pat. No. 5,155,336 assignedto the Assignee of the present application. Additionally, although theinsitu moisture generation reaction of the present invention ispreferably carried out in a rapid thermal heating apparatus, other typesof thermal reactors may be utilized such as the Epi or Poly Centurasingle wafer “cold wall” reactor by Applied Materials used to form hightemperature films (HTF) such as epitaxial silicon, polysilicon, oxidesand nitrides.

[0023]FIGS. 1 and 2 illustrate a rapid thermal heating apparatus 100which can be used to carry out the insitu moisture oxidation process ofthe present invention. Rapid thermal heating apparatus 100, as shown inFIG. 1, includes an evacuated process chamber 13 enclosed by a sidewall14 and a bottom wall 15. Sidewall 14 and bottom wall 15 are preferablymade of stainless steel. The upper portion of sidewall 14 of chamber 13is sealed to window assembly 17 by “O” rings 16. A radiant energy lightpipe assembly 18 is positioned over and coupled to window assembly 17.The radiant energy assembly 18 includes a plurality of tungsten halogenlamps 19, for example Sylvania EYT lamps, each mounted into a light pipe21 which can be a stainless steel, brass, aluminum or other metal.

[0024] A substrate or wafer 61 is supported on its edge in side chamber13 by a support ring 62 made up of silicon carbide. Support ring 62 ismounted on a rotatable quartz cylinder 63. By rotating quartz cylinder63 support ring 62 and wafer 61 can be caused to rotate. An additionalsilicon carbide adapter ring can be used to allow wafers of differentdiameters to be processed (e.g., 150 mm as well as 200 mm). The outsideedge of support ring 62 preferably extends less than two inches from theoutside diameter of wafer 61. The volume of chamber 13 is approximatelytwo liters.

[0025] The bottom wall 15 of apparatus 100 includes a gold coated topsurface 11 for reflecting energy onto the backside of wafer 61.Additionally, rapid thermal heating apparatus 100 includes a pluralityof fiber optic probes 70 positioned through the bottom wall 15 ofapparatus 100 in order to detect the temperature of wafer 61 at aplurality of locations across its bottom surface. Reflections betweenthe backside of the silicon wafer 61 and reflecting surface 11 create ablackbody cavity which makes temperature measurement independent ofwafer backside emissivity and thereby provides accurate temperaturemeasurement capability.

[0026] Rapid thermal heating apparatus 100 includes a gas inlet 69formed through sidewall 14 for injecting process gas into chamber 13 toallow various processing steps to be carried out in chamber 13. Coupledto gas inlet 69 is a source, such as a tank, of oxygen containing gassuch as O₂ and a source, such as a tank, of hydrogen containing gas suchas H₂. Positioned on the opposite side of gas inlet 69, in sidewall 14,is a gas outlet 68. Gas outlet 68 is coupled to a vacuum source, such asa pump, to exhaust process gas from chamber 13 and to reduce thepressure in chamber 13. The vacuum source maintains a desired pressurewhile process gas is continually fed into the chamber during processing.

[0027] Lamps 19 include a filament wound as a coil with its axisparallel to that of the lamp envelope. Most of the light is emittedperpendicular to the axis towards the wall of the surrounding lightpipe. The light pipe length is selected to at least be as long as theassociated lamp. It may be longer provided that the power reaching thewafer is not substantially attenuated by increased reflection. Lightassembly 18 preferably includes 187 lamps positioned in a hexagonalarray or in a “honeycomb shape” as illustrated in FIG. 2. Lamps 19 arepositioned to adequately cover the entire surface area of wafer 61 andsupport ring 62. Lamps 19 are grouped in zones which can beindependently controlled to provide for extremely uniform heating ofwafer 61. Heat pipes 21 can be cooled by flowing a coolant, such aswater, between the various heat pipes. The radiant energy source 18comprising the plurality of light pipes 21 and associated lamps 19allows the use of thin quartz windows to provide an optical port forheating a substrate within the evacuative process chamber.

[0028] Window assembly 17 includes a plurality of short light pipes 41which are brazed to upper/lower flange plates which have their outeredges sealed to an outer wall 44. A coolant, such as water, can beinjected into the space between light pipes 41 to serve to cool lightpipes 41 and flanges. Light pipes 41 register with light pipes 21 of theilluminator. The water cooled flange with the light pipe pattern whichregisters with the lamp housing is sandwiched between two quartz plates47 and 48. These plates are sealed to the flange with “O” rings 49 and51 near the periphery of the flange. The upper and lower flange platesinclude grooves which provide communication between the light pipes. Avacuum can be produced in the plurality of light pipes 41 by pumpingthrough a tube 53 connected to one of the light pipes 41 which in turnis connected to the rest of the pipes by a very small recess or groovein the face of the flange. Thus, when the sandwiched structure is placedon a vacuum chamber 13 the metal flange, which is typically stainlesssteel and which has excellent mechanical strength, provides adequatestructural support. The lower quartz window 48, the one actually sealingthe vacuum chamber 13, experiences little or no pressure differentialbecause of the vacuum on each side and thus can be made very thin. Theadapter plate concept of window assembly 17 allows quartz windows to beeasily changed for cleaning or analysis. In addition, the vacuum betweenthe quartz windows 47 and 48 of the window assembly provides an extralevel of protection against toxic gasses escaping from the reactionchamber.

[0029] Rapid thermal heating apparatus 100 is a single wafer reactionchamber capable of ramping the temperature of a wafer 61 or substrate ata rate of 25-100° C./sec. Rapid thermal heating apparatus 100 is said tobe a “cold wall” reaction chamber because the temperature of the waferduring the oxidation process is at least 400° C. greater than thetemperature of chamber sidewalls 14. Heating/cooling fluid can becirculated through sidewalls 14 and/or bottom wall 15 to maintain wallsat a desired temperature. For a steam oxidation process utilizing theinsitu moisture generation of the present invention, chamber walls 14and 15 are maintained at a temperature greater than room temperature(23° C.) in order to prevent condensation. Rapid thermal heatingapparatus 100 is preferably configured as part of a “cluster tool” whichincludes a load lock and a transfer chamber with a robotic arm.

[0030] A method of insitu generation of moisture or steam in a rapidthermal oxidation process according to the present invention isillustrated in flow chart 300 of FIG. 3. The method of the presentinvention will be described with respect to an insitu moisturegeneration process in the rapid thermal heating apparatus illustrated inFIGS. 1 and 2. Additionally, the oxidation process of the presentinvention will be described with respect to the steam oxidation orpassivation of a silicon gate electrode 402 and a silicon substratesurface 404 of a silicon wafer 61 as shown in FIG. 4a. It is to beappreciated that the insitu moisture generation oxidation process of thepresent invention can be used to oxidize any form of silicon includingepitaxial, amorphous, or polycrystalline, including doped and undopedforms. Additionally the process can be used to passivate or oxidizeother device or circuit features including but not limited to emitterand capacitor electrodes, interconnects and trenches, as well as be usedto form gate dielectric layers.

[0031] The first step according to the present invention, as set forthin block 302, is to move a wafer or substrate, such as wafer 61 intovacuum chamber 13. As is typical with modem cluster tools, wafer 61 willbe transferred by a robot arm from a load lock through a transferchamber and placed face up onto silicon carbide support ring 62 locatedin chamber 13 as shown in FIG. 1. Wafer 61 will generally be transferredinto vacuum chamber 13 having a nitrogen (N₂) ambient at a transferpressure of approximately 20 torr. Chamber 13 is then sealed.

[0032] Next, as set forth in block 304, the pressure in chamber 13 isfurther reduced by evacuating the nitrogen (N₂) ambient through gasoutlet 70. Chamber 13 is evacuated to a pressure to sufficiently removethe nitrogen ambient. Chamber 13 is pumped down to a prereactionpressure less than the pressure at which the insitu moisture generationis to occur, and is preferably pumped down to a pressure of less than 1torr.

[0033] Simultaneous with the prereaction pump down, power is applied tolamps 19 which in turn irradiate wafer 61 and silicon carbide supportring 62 and thereby heat wafer 61 and support ring 62 to a stabilizationtemperature. The stabilization temperature of wafer 61 is less than thetemperature (reaction temperature) required to initiate the reaction ofthe hydrogen containing gas and oxygen containing gas to be utilized forthe insitu moisture generation. The stabilization temperature in thepreferred embodiment of the present invention is approximately 500° C.

[0034] Once the stabilization temperature and the prereaction pressureare reached, chamber 13 is backfilled with the desired mixture ofprocess gas. The process gas includes a reactant gas mixture comprisingtwo reactant gasses: a hydrogen containing gas and an oxygen containinggas, which can be reacted together to form water vapor (H₂O) attemperatures between 400-1250° C. The hydrogen containing gas, ispreferably hydrogen gas (H₂), but may be other hydrogen containinggasses such as, but not limited to, ammonia (NH₃), deuterium (heavyhydrogen) and hydrocarbons such as methane (CH₄). The oxygen containinggas is preferably oxygen gas (O₂) but may be other types of oxygencontaining gases such as but not limited to nitrous oxide (N₂O). Othergasses, such as but not limited to nitrogen (N₂), may be included in theprocess gas mix if desired. The oxygen containing gas and the hydrogencontaining gas are preferably mixed together in chamber 13 to form thereactant gas mixture.

[0035] In the present invention the partial pressure of the reactant gasmixture (i.e., the combined partial pressure of the hydrogen containinggas and the oxygen containing gas) is controlled to ensure safe reactionconditions. According to the present invention, chamber 13 is backfilledwith process gas such that the partial pressure of the reactant gasmixture is less than the partial pressure at which spontaneouscombustion of the entire volume of the desired concentration ratio ofreactant gas will not produce a detonation pressure wave of apredetermined amount. The predetermined amount is the amount of pressurethat chamber 13 can reliably handle without failing. FIG. 5 is a graphwhich shows detonation pressures for different reactant gas mixtures ofO₂ and H₂ at a partial pressure of 150 torr for the spontaneouscombustion of the entire volume, about 2 liters, of chamber 13 at aprocess temperature of 950° C. According to the present invention,insitu moisture generation is preferably carried out in a reactionchamber that can reliably handle a detonation pressure wave of fouratmospheres or more without affecting its integrity. In such a case,reactant gas concentrations and operating partial pressure preferably donot provide a detonation wave greater than two atmospheres for thespontaneous combustion of the entire volume of the chamber.

[0036] By controlling the chamber partial pressure of the reactant gasmixture in the present invention any concentration ratio of hydrogencontaining gas and oxygen containing gas can be used including hydrogenrich mixtures utilizing H2/O₂ ratios greater than 2:1, respectively, andoxygen rich mixtures using H₂/O₂ ratios less than 0.5:1, respectively.For example, FIG. 5 shows that any concentration ratio of O₂ and H₂ canbe safely used as long as the chamber partial pressure of the reactantgasses is maintained at less than 150 torrs at process temperature. Theability to use any concentration ratio of oxygen containing gas andhydrogen containing gas enables one to produce an ambient with anydesired concentration ratio of H₂/H₂O or any concentration ratio ofO₂/H₂O desired. Whether the ambient is oxygen rich or dilute steam orhydrogen rich or dilute steam can greatly affect device electricalcharacteristics. The present invention enables a wide variety ofdifferent steam ambients to be produced and therefore a wide variety ofdifferent oxidation processes to be implemented.

[0037] In some oxidation processes, an ambient having a low steamconcentration with the balance O₂ may be desired. Such an ambient can beformed by utilizing a reactant gas mixture comprising 10% H₂ and 90% O₂.In other processes, an ambient of hydrogen rich steam (70-80% H₂/30-20%H₂O) may be desired. A hydrogen rich, low steam concentration ambientcan be produced according to the present invention by utilizing areactive gas mix comprising between 5-20% O₂ with the remainder H₂(95-80%). It is to be appreciated that in the present invention anyratio of hydrogen containing gas and oxygen containing gas may beutilized because the heated wafer provides a continual ignition sourceto drive the reaction. Unlike pyrogenic torch methods, the presentinvention is not restricted to specific gas ratios necessary to keep astable flame burning.

[0038] Next, as set forth in block 308, power to lamps 19 is increasedso as to ramp up the temperature of wafer 61 to process temperature.Wafer 61 is preferably ramped from the stabilization temperature toprocess temperature at a rate of between 10-100° C./sec with 50° C./secbeing preferred. The preferred process temperature of the presentinvention is between 600-1150° C. with 950° C. being preferred. Theprocess temperature must be at least the reaction temperature (i.e.,must be at least the temperature at which the reaction between theoxygen containing gas and the hydrogen containing gas can be initiatedby wafer 61) which is typically at least 600° C. It is to be noted thatthe actual reaction temperature depends upon the partial pressure of thereactant gas mixture as well as on the concentration ratio of thereactant gas mixture, and can be between 400° C. to 1250° C.

[0039] As the temperature of wafer 61 is ramped up to processtemperature, it passes through the reaction temperature and causes thereaction of the hydrogen containing gas and the oxygen containing gas toform moisture or steam (H₂O). Since rapid thermal heating apparatus 100is a “cold wall” reactor, the only sufficiently hot surfaces in chamber13 to initiate the reaction is the wafer 61 and support ring 62. Assuch, in the present invention the moisture generating reaction occursnear, about 1 cm from, the surface of wafer 61. In the present inventionthe moisture generating reaction is confined to within about two inchesof the wafer, or about the amount at which support ring 62 extends pastthe outside edge of wafer 61. Since it is the temperature of the wafer(and support ring) which initiates or turns “on” the moisture generationreaction, the reaction is said to be thermally controlled by thetemperature of wafer 61 (and support ring 62). Additionally, the vaporgeneration reaction of the present invention is said to be “surfacecatalyzed” because the heated surface of the wafer is necessary for thereaction to occur, however, it is not consumed in the reaction whichforms the water vapor.

[0040] Next, as set forth in block 310, once the desired processtemperature has been reached, the temperature of wafer 61 is heldconstant for a sufficient period of time to enable the water vaporgenerated from the reaction of the hydrogen containing gas and theoxygen containing gas to oxidize silicon surfaces or films to form SiO₂.Wafer 61 will typically be held at process temperature for between30-120 seconds. Process time and temperature are generally dictated bythe thickness of the oxide film desired, the purpose of the oxidation,and the type and concentrations of the process gasses. FIG. 4billustrates an oxide 406 formed on wafer 61 by oxidation of siliconsurfaces 402 and 404 by water vapor (H₂O) generated by the insitumoisture generation process of the present invention. It is to beappreciated that the process temperature must be sufficient to enablethe reaction of the generated water vapor or steam with silicon surfacesto form silicon dioxide.

[0041] Next, as set forth in block 312, power to lamps 19 is reduced orturned off to reduce the temperature of wafer 61. The temperature ofwafer 61 decreases (ramps down) as fast as it is able to cool down (atabout 50° C./sec.). Simultaneously, N2 purge gas is fed into the chamber13. The moisture generation reaction ceases when wafer 61 and supportring 62 drop below the reaction temperature. Again it is the wafertemperature (and support ring) which dictates when the moisture reactionis turned “on” or “off”.

[0042] Next, as set forth in block 314, chamber 13 is pumped down,preferably below 1 torr, to ensure that no residual oxygen containinggas and hydrogen containing gas are present in chamber 13. The chamberis then backfilled with N₂ gas to the desired transfer pressure ofapproximately 20 torr and wafer 61 transferred out of chamber 13 tocomplete the process. At this time a new wafer may be transferred intochamber 13 and the process set forth in flow chart 300 repeated.

[0043] At times it may be desirable to utilize concentration ratios ofhydrogen containing gas and oxygen containing gas which will produce anambient with a large concentration of water vapor (e.g., >40% H₂O). Suchan ambient can be formed with a reactant gas mixture, for example,comprising 40-80% H₂/60-20% O₂. A gas mixture near the stoichiometricratio may yield too much combustible material to enable safe reactionconditions. In such a situation, a low concentration gas mixture (e.g.,less than 15% O₂ in H₂) can be provided into the reaction chamber duringstep 306, the wafer temperature raised to the reaction temperature instep 308, and the reaction initiated with the lower concentration ratio.Once the reaction has been initiated and the existing reactant gasvolume begins to deplete, the concentration ratio can be increased tothe desired level. In this way, the amount of fuel available at thestart of the reaction is kept small and safe operating conditionsassured.

[0044] In an embodiment of the present invention a relatively low,reactive gas partial pressure is used for insitu steam generation inorder to obtain enhanced oxidation rates. It has been found thatproviding a partial pressure of between 1 Torr to 50 Torr of hydrogengas (H₂) and oxygen gas (O₂) that an enhanced oxide growth rate ofsilicon can be achieved. That is, for a given set of process conditions(i.e., H₂/O₂ concentration ratio, temperature, and flow rate) theoxidation rate of silicon is actually higher for lower partial pressures(1-50 Torr) of H₂ and O₂ than for higher partial pressures (i.e., from50 Torr to 100 Torr).

[0045] The plots of FIG. 6 illustrate how reactant gas partial pressurescan enhance the oxidation rate of silicon. Plot 602 depicts differentoxide thicknesses that are formed for different reactant gas partialpressures for an ambient created by reacting 9% H₂ with 91% O₂ at 1050°C. for 30 seconds. Plot 604 depicts different oxide thicknesses that areformed for different reactant gas partial pressures for an ambientcreated by reacting 33% H₂ with 66% O₂ at 1050° C. for 60 seconds.

[0046] As is apparent from the graphs of FIG. 6, as the reactant gaspartial pressure of H₂ and O₂ is incrementally decreased fromatmospheric pressure to about 50 Torr for 9% H₂, and to about 30 Torrfor 33% H₂, the oxidation rate of silicon also decreases incrementally.A decrease in oxidation rate for silicon with a decrease in reactant gaspartial pressure is expected in that one would expect when less O₂ andH₂ are available for the generation of steam the oxidation rate woulddecrease. When a reactant gas partial pressure of less than or equal toapproximately 50 Torr for 9% H₂ and 30 Torr for 33% H₂ obtained,however, the oxidation rate quite unexpectedly begins to increase withincremental decreases in reactant gas partial pressure. The oxidationrate continues to increase until a maximum enhanced oxidation rate isreached at approximately 8-12 Torr at which point the oxidation ratebegins to decrease for incremental decreases in reactant gas partialpressure. Although the oxidation rate begins to decrease after themaximum enhanced oxidation rate achieved at 8-12 Torr, it still providesan enhanced oxidation rate (i.e., provides an oxidation rate greaterthan the oxidation rate generated at approximately 50 Torr (9% H₂) and30 Torr (33% H₂)) until a reactant gas partial pressure of approximately1-3 Torr at which point the oxidation rate enhancement falls off.

[0047] In the enhanced oxidation embodiment of the present invention,the insitu steam generation is carried out at a reactant gas partialpressure of oxygen containing gas and hydrogen containing gas whereenhanced oxidation occurs. That is, in an embodiment of the presentinvention insitu steam generation occurs at a maximum reactant gaspartial pressure of oxygen gas (O₂) and hydrogen gas (H₂) which is lessthan or equal to the reactant gas partial pressure at which a decreasein reactant gas partial pressure for a given set of process parameterscauses an increase in the oxidation rate of silicon. Additionally in theenhanced oxidation embodiment of the present invention the minimumreactant gas partial pressure is that at which the oxidation rate isgreater than or equal to the oxidation rate at the maximum reactant gaspartial pressure. The minimum reactant gas partial pressure is generallyabout 1-3 Torr. Enhanced oxidation of silicon by a steam ambient can becreated by reacting H₂ and O₂ between a minimum reactant gas partialpressure of between 1-3 Torr and a maximum reactant gas partial pressureof about 50 Torr. In another embodiment of the present invention insitusteam generation is carried out at a combined oxygen containing gas andhydrogen containing gas partial pressure of between 5-15 Torr which iswhere the peak enhanced oxidation rate occurs.

[0048] It is to be appreciated that operating the insitu steamgeneration process of the present invention at a reactant gas partialpressure at which enhanced oxidation occurs is valuable for a number ofreasons. An increase in oxidation rate means less oxidation time isrequired to grow an oxide of a given thickness which increasesthroughput which thereby decreases the cost of ownership of a tool. Suchincreases in wafer throughput are extremely important when single waferreactors such as rapid thermal heating apparatus 100 are utilized.Additionally short oxidation times reduce the thermal budget ofsemiconductor chips, which improves their performance and reliability.Additionally, an increase in oxidation rate enables the insitu steamgeneration process of the present invention to be used for thegeneration of thick oxides (e.g., oxides greater than 100 Å).

[0049] Still further operating at low reactant gas partial pressures notonly provides the advantage of enhanced oxidation rate but also providesthe advantage of safe operating conditions in that the detonationpressure created by the spontaneous combustion of the entire volume ofthe chamber is minimized due to the small amount of fuel available.Additionally, operating at low partial pressures prevents thecondensation of moisture inside a “cold wall” chamber which prevents theintroduction of an uncontrolled reactant.

[0050] Although the oxidation rate of only two concentration ratios ofH₂/O₂ are illustrated in FIG. 6, the oxidation rate of otherconcentration ratios between 2% H₂/98% O₂ to 66% H₂/33% O₂ behavesimilarly. It has been found that when operating at reactant gas partialpressures where enhanced oxidation occurs, that the oxidation rate ofsilicon is influenced by the concentration ratio of the hydrogencontaining gas and the oxygen containing gas. For example, FIG. 7illustrates different oxidation thicknesses for different concentrationratios of H₂ and O₂ for a given set of process parameters (i.e., O₂ flow10 SLM, reactant gas partial pressure 10 Torr, temperature 1050° C., andtime 30 seconds). As illustrated in FIG. 7, the greatest increase inoxidation rate occurs between 1-5% H₂ while after 33% H₂ the oxidationrate stabilizes at about 150 Å per minute.

[0051]FIG. 8 illustrates how oxide thickness varies for oxidation timefor different insitu steam oxidation processes (33% H₂/66% O₂; 5% H₂/95%O₂; 2% H₂/98% O₂; or at 10 Torr) and different dry oxidation processes(100% O₂ at 10 Torr and 100% O₂ at atmospheric). As illustrated in FIG.8, reduced pressure steam oxidation processes provide for increasedoxidation rates over dry oxidation processes at the same pressure.Additionally, insitu steam generated oxidation processes with a H₂concentration greater than 3% provide higher oxidation rates than do dryoxidation processes at all oxidation pressures including atmosphericpressure.

[0052] In an embodiment of the present invention, a concentration ratiobetween 2% H₂/98% O₂ to 33% H₂/66% O₂ is utilized because such producesa sufficient oxidation rate but yet utilizes a low concentration ofreactant gas which makes the process safe. It is to be appreciated thatwhen concentration ratios are closer to the stoichiometric ratio (66%H₂/33% O₂) there is the potential for the entire volume of the chamberto spontaneously combust. By operating in the concentration range ofbetween 2%-33% H₂, one is able to obtain oxidation rates near theoxidation rate of the stoichiometric ratio but without the danger of thespontaneous combustion of the entire volume since only a smallpercentage of H₂ is available for reaction. It has been found that byoperating with a concentration ratio of 33% H₂/66% O₂, a good oxidationrate can be obtained while providing a sufficiently low concentration ofH₂ to ensure safe operating conditions.

[0053] When operating at oxidation pressures which obtain enhancedsilicon oxidation rates, the oxidation rate is strongly influenced bythe total flow rate of the oxygen containing gas and the hydrogencontaining gas. For example, FIG. 9 illustrates how the oxidation rateof silicon varies for the total flow rate of a 33% H₂/66% O₂ reactantgas mix at a reactant gas partial pressure of 10 Torr and a temperatureof 1050° C. in rapid thermal processing apparatus 100, having a chambervolume of approximately 2 liters. As shown in FIG. 9, when operating atlow reactant gas partial pressures, in order to generate enhancedoxidation rates, an increase in the total flow increases the oxidationrate. As shown in FIG. 9 the oxidation rate increases dramatically foran increase in total flow when the total flow is less than 10 SLM andincreases, but less dramatically, for increases in total flow above 10SLM.

[0054] Accordingly, when operating at a partial pressure to provideenhanced oxidation, the oxidation rate of silicon can be said to be“mass transport rate” limited. That is, the oxidation rate is limited bythe amount of reactant gas fed into the chamber. The fact that theinsitu steam oxidation process of the present invention can be “masstransport rate” limited is quite unexpected in that the presentinvention utilizes relatively large reactant gas flow rates (greaterthan 5 SLM into a 2 liter chamber) At such high flow rates one wouldexpect there to be sufficient reactants available to make the oxidationrate independent of the mass transport rate. It is to be appreciatedthat silicon oxidation processes are generally thought to be “surfacereaction rate” limited where the temperature controls the oxidation rateand not the flow rate of the reactant gases.

[0055] Although the present invention has been described with respect tothe insitu generation of a vapor of a specific reactive species, water,it is to be appreciated that the teachings of the present invention canbe applied to other processes where the temperature of a wafer is usedto initiate or catalyze the reaction of reactant gasses to form a vaporof a reactive species near the wafer surface. The reactive species vaporcan then be reacted with the wafer or with films formed thereon to carryout processes such as film growth. For example, the insitu vaporgeneration process of the present invention can be utilized to convert asilicon dioxide (SiO₂) film into a robust silicon-oxy-nitride film. Forexample, a reactant gas mixture comprising ammonia (NH₃) and oxygen (O₂)can be fed into a chamber and then caused to react by heating a wafer toa sufficient temperature to initiate a reaction of the gasses to formnitric oxide (NO) in vapor form. The nitric oxide vapor can then becaused to react with an oxide film formed on the wafer to form asilicon-oxy-nitride film. Silicon-oxy-nitride films have been found toprovide robust gate dielectric layers at thicknesses less than 100 Å.Other applications for the insitu vapor generation process of thepresent invention will be evident to those skilled in the art.

[0056] Thus, a novel method and apparatus for the insitu generation ofsteam in a rapid thermal oxidation process has been described.

We claim:
 1. A method of forming an oxide, said method comprising thesteps of: placing a substrate in a chamber; providing an oxygencontaining gas into said chamber; providing a hydrogen containing gasinto said chamber; reacting said oxygen containing gas and said hydrogencontaining gas in said chamber to form water vapor (H₂O) in saidchamber; and oxidizing said substrate with said water.
 2. The method ofclaim 1 further comprising the step of: heating said substrate to atemperature sufficient to initiate said reaction between said oxygencontaining gas and said hydrogen containing gas.
 3. The method of claim2 wherein said substrate is heated to a temperature of at least 600° C.4. The method of claim 1 further comprising the step of generating acombined partial pressure of said oxygen containing gas and saidhydrogen containing gas in said chamber of less than one atmosphereprior to reacting said oxygen containing gas and said hydrogencontaining gas.
 5. The method of claim 4 wherein the combined partialpressure of said hydrogen containing gas and said oxygen containing gasis less than or equal to 150 torr during said reaction.
 6. The method ofclaim 1 wherein said oxygen containing gas is oxygen (O₂).
 7. The methodof claim 1 wherein said oxygen containing gas is nitrous oxide (N₂O). 8.The method of claim 1 wherein said hydrogen containing gas is ahydrocarbon.
 9. The method of claim 1 wherein said hydrogen containinggas is hydrogen gas (H₂).
 10. The method of claim 1 wherein saidhydrogen containing gas is ammonia (NH₃).
 11. The method of claim 1wherein said hydrogen containing gas is a hydrocarbon.
 12. A method offorming an oxide, said method comprising the steps of: placing asubstrate having a silicon film in a chamber; providing an oxygencontaining gas into said chamber; providing a hydrogen containing gasinto said chamber; heating said substrate to a temperature sufficient tocause a reaction between said hydrogen containing gas and said oxygencontaining gas; reacting said oxygen containing gas and said hydrogencontaining gas in said chamber to form water vapor (H₂O) wherein saidreaction is initiated by said heated substrate; and oxidizing saidsilicon film with said water vapor to form said oxide.
 13. The method ofclaim 12 further comprising the step of generating a combined partialpressure of said oxygen containing gas and said hydrogen containing gasin said chamber of less than one atmosphere prior to reacting saidoxygen containing gas and said hydrogen containing gas.
 14. The methodof claim 13 wherein the combined partial pressure of said hydrogencontaining gas and said oxygen containing gas is less than or equal to150 torr during said reaction.
 15. The method of claim 12 wherein saidheating step heats said substrate to a temperature of at least 600° C.16. A method of forming an oxide, said method comprising the steps of:placing a substrate having a silicon film in a reaction chamber;providing an oxygen containing gas into said chamber; providing ahydrogen containing gas into said chamber; generating a combined partialpressure of said oxygen containing gas and said hydrogen containing gasof less than one atmosphere; heating said substrate; reacting saidoxygen containing gas and said hydrogen containing gas in said chamberto form water vapor (H₂O), wherein said reaction is initiated by saidheated substrate; and oxidizing said silicon film with said water vaporto form said oxide.
 17. The method of claim 16 wherein said combinedpartial pressure is less than or equal to 150 torr while reacting saidoxygen containing gas and said hydrogen gas (H₂).
 18. The method ofclaim 16 wherein said wafer is heated to a temperature of at least 600°C.
 19. A method of growing an oxide layer in a rapid thermal processor,said method comprising the steps of: placing a silicon wafer having asilicon film in a reaction chamber of said rapid thermal processor;heating said substrate to a temperature greater than or equal to 700° C;providing oxygen gas (O₂) into said chamber; providing hydrogen gas (H₂)into said chamber; generating a combined partial pressure of said oxygencontaining gas and said hydrogen containing gas of less than or equal to150 torr in said chamber; reacting said oxygen gas and said hydrogen gasin said chamber to form water vapor (H₂O) in said chamber, wherein saidreaction is initiated by said heated substrate; and oxidizing saidsilicon film and said silicon wafer with said water vapor (H₂O).
 20. Amethod of processing a substrate, said method comprising the steps of:placing a substrate in a chamber; providing a first and a secondreactant gas into said chamber; and heating said substrate, wherein saidheated substrate initiates a reaction between said first and said secondreactant gasses to form a reactive species.
 21. A method of forming anoxide, said method comprising the steps of: placing a substrate in achamber; providing an oxygen containing gas into said chamber; providinga hydrogen containing gas into said chamber; generating a combinedpartial pressure of said oxygen containing gas and said hydrogencontaining gas in said chamber of between 1 Torr and 50 Torr; reactingsaid oxygen containing gas with said hydrogen containing gas in saidchamber near the surface of said substrate to form an ambient; andoxidizing said substrate with said ambient.
 22. The method of claim 21wherein said combined partial pressure of said oxygen containing gas andsaid hydrogen containing gas in said chamber is between 5-15 Torr.
 23. Amethod of forming an oxide, said method comprising the steps of: placinga substrate in a reaction chamber; heating said substrate; providingoxygen gas (O₂) into said chamber and providing hydrogen gas (H₂) intosaid chamber wherein said hydrogen gas and said oxygen gas are providedin a hydrogen gas to oxygen gas ratio between 2:98 and 2:1 respectively;generating a combined partial pressure of said oxygen gas (O₂) and saidhydrogen gas (H₂) of between 1 Torr and 50 Torr; reacting said oxygengas (O₂) and said hydrogen gas (H₂) in said chamber near said substrateto form an ambient in said chamber wherein said reaction is initiated bysaid heated substrate; and oxidizing said substrate with said watervapor.
 24. The method of claim 23 wherein said combined partial pressureof said oxygen gas (O₂) and said hydrogen gas (H₂) is between 5-15 Torr.25. The method of claim 23 wherein said ratio of hydrogen gas (H₂) tooxygen gas (O₂) is between 2:98 and 1:2.
 26. The method of claim 25wherein said ratio of said hydrogen gas to said oxygen gas isapproximately 33:66, respectively.
 27. A method of forming an oxide,said method comprising the steps of: placing the substrate in a chamber;providing an oxygen containing gas into said chamber; providing ahydrogen containing gas into said chamber; generating a partial pressureof said oxygen containing gas and said hydrogen containing gas in saidchamber; reacting said oxygen containing gas with said hydrogencontaining gas in said chamber near the surface of said substrate toform an ambient; oxidizing said substrate with said ambient; and whereinsaid partial pressure is less than or equal to the partial pressure atwhich a decrease in said partial pressure causes an increase in theoxidation rate of said substrate by said ambient;
 28. A method offorming an oxide, said method comprising the steps of: placing thesubstrate in a chamber; providing an oxygen containing gas into saidchamber; providing a hydrogen containing gas into said chamber;generating a partial pressure of said oxygen containing gas and saidhydrogen containing gas in said chamber; reacting said oxygen containinggas with said hydrogen containing gas in said chamber near the surfaceof said substrate to form an ambient; oxidizing said substrate with saidambient; and wherein at said partial pressure an incremental decrease inpartial pressure results in an increase in oxidation rate of saidsubstrate by said ambient.
 29. A method of forming an oxide, said methodcomprising the steps of: placing the substrate in a chamber; providingan oxygen containing gas into said chamber; providing a hydrogencontaining gas into said chamber; and generating an oxygen containinggas and hydrogen containing gas partial pressure and concentration ratiosuch that the oxidation rate of said substrate by an ambient created byreacting said oxygen containing gas and said hydrogen containing gas isgreater than the oxidation rate of said substrate by an ambient createdby reacting said concentration ratio of said hydrogen containing gas andsaid oxygen containing gas at a 100 Torr partial pressure of said oxygencontaining gas and said hydrogen containing gas.
 30. A method of formingan oxide, said method comprising the steps of: placing a substrate in achamber; providing a flow of oxygen containing gas and hydrogencontaining gas into said chamber; generating a partial pressure of saidoxygen containing gas and said hydrogen containing gas; reacting saidoxygen containing gas with said hydrogen containing gas in said chambernear the surface of said substrate to form an ambient; oxidizing saidsubstrate with said ambient; and wherein at said partial pressure theoxidation rate of silicon is “mass transport rate” limited.